Mechanisms that contribute to adhesion at the interface between two solids include polar and non-polar dispersion, hydrogen bonding, covalent and metallic bonding, charged bilayers, cross-linking, polymer entanglement, and mechanical locking. Many manufacturing bonding processes rely on these mechanisms, including painting, printing, plating, adhering, soldering, and spinning. Measurements of surface energy are often used to assess if a surface has been suitably prepared for bonding.
The surface free energy of a solid is typically defined as half the energy per unit area required to separate a solid into two half planes separated by vacuum. U.S. Pat. No. 5,477,732 describes bringing a characterized solid atomic force microprobe (AFM) into intimate contact with a surface under test, and then measuring the energy required to separate the probe and test surface. The high curvature of the probe tip makes the technique insensitive to most roughness of the test surface. Small AFM tips are generally formed from relatively hard, high surface energy materials.
Attempting a similar touch probe between a probe and test surface on a more convenient larger scale encounters the problem that most solid surfaces are somewhat rough and unclean. Most of the above mentioned forces are very short range, so that roughness that separates the two surfaces by a few Angstroms on average will reduce the measured force of attraction by more than an order of magnitude.
While small scale roughness of the surface to be adhered is generally an impediment to measuring the surface energy of that surface, it often improves bonding of the manufactured article. The standard approach to probe rough surfaces is to use a liquid that wets the rough surface to some degree. The surface tension of a liquid is the analog of the surface free energy of a solid. In particular, measuring the contact angle of a sessile droplet on the test surface has been related to the surface free energy of the test surface using relations like the Young-Dupre equation. Related liquid contact angle measurements include the Wilhelmy plate method, the fiber contact angle method, the pendant drop method, and the Du Nouy ring method. Applying droplets of different composition increase the range of measurable surface energies, and allow some differentiation between component contributions from dispersion, polar, and hydrogen bonding.
Surface roughness remains a problem for contact angle measurements. This is observed in several ways. Since the slope of the test surface is not constant along wetting line around a sessile drop, the wetting line can be a scalloped circle instead of a smooth circle. Typically the drying contact angle is smaller than the wetting contact angle, in part because surface roughness can generate a hysteresis that tends to pin the wetting line. The microscopic contact angle at the wetting front can be different from the measurable macroscopic contact angle.